Stereodivergent Anion Binding Catalysis with Molecular Motors

Article abstract of DOI:10.1002/anie.201913054

A photoresponsive chiral catalyst based on an oligotriazole-functionalized unidirectional molecular motor has been developed for stereodivergent anion binding catalysis. The motor function controls the helical chirality of supramolecular assemblies with chloride anions, which by means of chirality transfer enables the enantioselective addition of a silyl ketene acetal nucleophile to oxocarbenium cations. Reversal of stereoselectivity (up to 142 % Δee) was achieved through rotation of the motor core induced by photochemical and thermal isomerization steps.

Full text of DOI:10.1002/anie.201913054

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Accepted Article
Title: Stereodivergent Anion Binding Catalysis with Molecular Motors
Authors: Ruth Dorel and Ben Lucas Feringa
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10.1002/anie.201913054
Angewandte Chemie International Edition
RESEARCH ARTICLE
Stereodivergent Anion Binding Catalysis with Molecular Motors
Ruth Dorel[a] and Ben L. Feringa*[a]
Abstract: A photoresponsive chiral catalyst based on an oligotriazole-
functionalized unidirectional molecular motor has been developed for
stereodivergent anion binding catalysis. The motor function controls
the helical chirality of supramolecular assemblies with chloride anions,
which by means of chirality transfer enables the enantioselective
addition of a silyl ketene acetal nucleophile to oxocarbenium cations.
Reversal of stereoselectivity (up to 142% Δee) was achieved through
rotation of the motor core induced by photochemical and thermal
isomerization steps.
catalysis, which is achieved by means of supramolecular transfer
of chirality (Figure 1). The precise positioning of two ion binders
with respect to each other at each stage of the rotation cycle of
the motor creates different chiral environments at the anion
recognition site, which translates into different stereochemical
outcomes in anion binding catalysis.
Introduction
The allosteric regulation of enzymes in Nature[1] has served as a
source of inspiration for the development of responsive artificial
catalysts whose function can be altered by the action of external
stimuli,[ 2 , 3 ] which may translate into variations in activity,[ 4 ]
selectivity,[5] or reaction type[6] without reengineering the structure
of the catalyst. Particularly desirable and at the same time
remarkably challenging are switchable catalysts that can on-
demand selectively provide all the different stereoisomers in a
given chemical transformation.[ 7 ] Our group pioneered this
concept[8] through the use of so-called first generation molecular
motors[9] – which can be interconverted between four different
states by the action of light and heat – containing two reactive
sites attached to the motor core. Based on this approach, the
stereodivergent addition of a thiolate to an α,β-unsaturated
ketone was initially accomplished,[8] followed by the development
of thiourea-based switchable organocatalysts for the Henry
reaction[10] as well as a Trost-type ligand for a palladium-catalyzed
asymmetric allylic substitution.[11] More recently, photoswitchable
catalysts based on second generation molecular motors[12] have
also been realized.[13]
Figure 1. Approach to the use of molecular motors for stereodivergent anion
binding catalysis.
Triazoles have been recognized as amide surrogate anion
binders due to their dipole moment (~5 D), which results in an
electropositive C–H capable of engaging in hydrogen bonding
with negatively charged species.[17] In the case of aryl triazole
oligomers that cannot adopt
interaction translates into a helical supramolecular arrangement
in the presence of, among others, chloride anions.[18,19] Based on
a planar conformation, this
Asymmetric anion binding catalysis is based on the activation of
an ion pair for the attack of a nucleophile through the recognition
of the anion by a chiral receptor.[14] Since the groundbreaking
reports by Jacobsen and coworkers on the use of chiral thioureas
as the anion binding catalysts,[15] several other classes of anion
receptors have been designed and successfully applied in
asymmetric catalysis.[ 16 ] Herein, we report on the use of a
molecular motor-based stimuli-responsive anion receptor to
illustrate the concept of adaptive stereodivergent anion binding
this concept, chiral oligotriazoles bearing
a
trans-1,2-
cyclohexanediamine backbone have been developed[20] and their
utility in asymmetric anion binding catalysis has been elegantly
illustrated for the dearomatization of a variety of heterocycles.[21]
We anticipated that a first generation molecular motor could serve
as a unique responsive chiral scaffold for oligotriazole-based
anion receptors, which due to the motor function could
interconvert between isomeric states as a response to external
stimuli. In our design, molecular motors 1 feature two branches,
each of them containing two triazole moieties linked by an aryl
group (Scheme 1). While the two branches would be far apart
from each other in (R,R)-(P,P)-trans-1, they could come into close
proximity in both (R,R)-(M,M)-cis-1 and (R,R)-(P,P)-cis-1 forms,
which would be sequentially accessed by irradiation of (R,R)-
(P,P)-trans-1 and heating of the resulting photogenerated species,
respectively. These species are pseudoenantiomers, only
[a]
R. Dorel, B. L. Feringa
Stratingh Institute for Chemistry, Zernike Institute for Advanced
Materials, University of Groningen, Nijenborgh 4, 9747AG
Groningen (The Netherlands)
E-mail: b.l.feringa@rug.nl
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.201913054
Angewandte Chemie International Edition
RESEARCH ARTICLE
differing from each other in the handedness of the helical structure. from cis-2 turned out to be unsuccessful, the double aromatic
In the presence of chloride anions, (R,R)-(M,M)-cis-1 and (R,R)-
(P,P)-cis-1 can form a supramolecular helical assembly to give
Finkelstein reaction[25] proceeded smoothly on cis-2 to afford cis-
3, which could be coupled with trimethylsilylacetylene under
rise to (R,R)-(M,M)-cis-1-Cl and (R,R)-(P,P)-cis-1-Cl, respectively, standard reaction conditions. Subsequent cleavage of the
whose configuration would be dictated by the helicity of the motor
backbone.[22] We envisioned that this transfer and amplification of
chiral information from the molecular to the supramolecular level
could be exploited to achieve the preferential formation of different
stereoisomers in the context of asymmetric anion binding
catalysis depending upon the external stimulus applied.
trimethylsilyl groups at the alkyne termini provided cis-4, which
was obtained in its enantiomerically pure form after
recrystallization. Alternatively, the double aromatic Finkelstein
followed by double Sonogashira coupling starting from the
mixture of cis- and trans-2 provided separable mixtures of bis-
alkynylated motors, which after TMS cleavage afforded cis- and
trans-4. Ultimately, the reaction of cis-4 with aromatic azides 5
and 6 in the presence of substoichiometric amounts of copper
afforded tetra- and bistriazole-functionalized molecular motors
cis-1a-d and cis-7, respectively. In the case of cis-1a-d, different
electron withdrawing substituents (R) at the ring connecting the
triazole moieties were selected, since they are known to have an
impact on the performance of oligotriazole receptors in
asymmetric catalysis.[20] Likewise, trans-4 could be used to obtain
trans-configured oligotriazole-containing molecular motors 1.
CF3
CF3
N
F3C
N
N
N
N
R
N
CF3
N
N
N
N
N
N
R
(R,R)-(P,P)-trans-1
F3C
CF3
312 nm
365 nm
F3C
CF3
N
1)
TMS
N
N
CuI,
N
N
PdCl2(PPh3)2, CuI
TEA, 50 ºC, 16 h
79%
I
Cl-
N
Br
MeHN
NHMe
N
N
N
N
NaI, dioxane
140 ºC, 24 h
75%
2) TBAF, THF
0 ºC, 1 h
99%
R
N
N
I
Br
R
N
N
N
N
R
312 nm
N
heptane
180 ºC, 5 h
99%
cis-2:trans-2 = 4:1
cis-3
F3C
N
CF3
cis-2:trans-2 > 20:1
R
N
N
N
CF3
R
N
(R,R)-(M,M)-cis-1
N
F3C
N
N
N
N
N
N
(R,R)-(M,M)-cis-1-Cl
N
CF3
CF3
N3
N
Stereodivergent
anion binding catalysis
(pseudoenantiomers)
5
N
N
F3C
(2.5 equiv) F3C
R
F3C
CF3
N
N
Δ
CuSO4·5H2O (1 equiv)
ascorbic acid (3 equiv)
tBuOH/DCM/H2O 1:1:2
N
F3C
CF3
R
N
N
N
Cl-
cis-4
N
N
N
e.r. > 99% (recryst)
N
N
N
N
F3C
CF3
N
N
N
CF3
N
F3C
N
N
R
R
6
N
N
N
N3
F3C
N
N
N
N
N
F3C
R
R
CuSO4 5H2O (1 equiv)
ascorbic acid (3 equiv)
tBuOH/DCM/H2O 1:1:2
66%
cis-1a: R = H, 64%
cis-1b: R = CF3, 68%
cis-1c: R = CO2Me, 61%
N
N
N
N
(R,R)-(P,P)-cis-1
N
N
CF3
F3C
N
OMe
cis-1d: R =
cis-7
N
N
62%
F3C
(R,R)-(P,P)-cis-1-Cl
CF3
CF3
F3C
Scheme 2. Synthesis of oligotriazole-functionalized motors (R,R)-(P,P)-cis-1a-
d and (R,R)-(P,P)-cis-7. TEA = triethylamine. TBAF = tetrabutylammonium
fluoride. THF = tetrahydrofurane. DCM = dichloromethane.
Scheme 1. Oligotriazole-functionalized molecular motors for stereodivergent
anion binding catalysis.
With enantiomerically pure (P,P)-cis-configured triazole-
containing motors 1a-d and 7 in hand, their performance as anion
binding catalysts was next examined. We selected the addition of
silyl ketene acetal nucleophiles to 1-chloroisochroman as a model
reaction (Table 1), which has been studied in the presence of
thiourea-based anion binders[15,26] among others.[27] We initially
tested the reaction between 1-chloroisochroman (8) and tert-
butyl((1-isopropoxyvinyl)oxy)dimethylsilane (9) in MTBE at -70 ºC
for 36 h. We found that two triazole moieties per branch are
Results and Discussion
Our starting point for the synthesis of the target triazole-
functionalized molecular motors was a mixture of cis- and trans-2
(e.r. > 95:5), which results from the McMurry coupling of the
corresponding enantiomerically enriched indanone.[
This
23
]
mixture could be converted into pure cis-2 by heating at 180 ºC
(Scheme 2).[24] While the attempts of direct Sonogashira coupling
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10.1002/anie.201913054
Angewandte Chemie International Edition
RESEARCH ARTICLE
required in order to induce enantioselectivity in this transformation
since (R,R)-(P,P)-cis-7 provided rac-10 (Table 1, entry 1), which
is in line with the predicted formation of a supramolecular helical
structure upon chloride binding. Among the tetratriazole-motors
(R,R)-(P,P)-cis-1a-d tested (Table 1, entries 2-5), motor (R,R)-
(P,P)-cis-1b bearing CF3 substituents at the linking ring gave the
highest conversion (65%) and enantioinduction (e.r. = 86:14) in
the formation of 10a (Table 1, entry 3) and was therefore selected
for further optimization studies. The use of other solvents
including Et2O, THF, CH2Cl2, or toluene resulted in lower values
of enantioselectivity (see Table S1 in the Supporting Information
for details). We also examined the influence of the nucleophile,
finding lower levels of enantioinduction with the other silyl ketene
acetals tested and no conversion when an enol ether was used
as the nucleophile (see Table S2 in the Supporting Information for
details). Therefore, 9 was the nucleophile of choice for further
experimentation. Lowering the reaction temperature to -80 ºC led
to an increase in e.r. (Table 1, entry 6). Finally, dilution to an initial
0.15 M concentration of 8a provided the addition product 10a with
90:10 e.r. (Table 1, entry 7), which was isolated in 81% yield after
extending the reaction time to 5 d (Table 1, entry 8). The
enantiomer preferentially formed in the presence of (R,R)-(P,P)-
cis-1b as the catalyst was determined to be (S)-10a by
comparison to previously published data (see Supporting
Information for details).[15]
Figure S1). This process could be reversed by irradiation with 365
nm light. Subsequent heating of (R,R)-(M,M)-cis-1b at 60 ºC
induced the thermal helix inversion (THI) to form (R,R)-(P,P)-cis-
1
1b. The same process was monitored by H NMR (Figure 1b).
Thus, a PSS ratio of 90:10 (R,R)-(M,M)-cis-1b:(R,R)-(P,P)-trans-
1b was obtained after irradiation of a solution of (P,P)-trans-1b in
d8-THF (312 nm) for 2 h. Both isomers could be easily separated
by preparative thin layer chromatography. After THI, the 1H NMR
spectrum obtained for (R,R)-(P,P)-cis-1b was identical to that of
the corresponding synthetic sample. Additionally, the effect of the
presence of chloride anions was studied by CD spectroscopy
(Figure 1c). The addition of 10 equiv of TBACl to a THF solution
of (R,R)-(P,P)-cis-1b (black line) and (R,R)-(M,M)-cis-1b (blue
line) resulted in an enhanced CD signal with negative ((R,R)-
(P,P)-cis-1b, red line) and positive ((R,R)-(M,M)-cis-1b, green
line) signs between 300 and 400 nm, which is indicative of the
formation of supramolecular helical structures with opposite
configurations upon anion binding.[22] Job plot analysis indicated
a 1:1 binding stoichiometry for the binding of chloride ions to the
three isomers of 1b (see Supporting Information for details).
c)
a)
(R,R)-(P,P)-cis-1b
(P,P)-trans-1b
(M,M)-cis-1b
(P,P)-cis-1b
(R,R)-(P,P)-cis-1b-Cl
(R,R)-(M,M)-cis-1b
(R,R)-(M,M)-cis-1b-Cl
θ
Table 1. Catalytic activity of motors (R,R)-(P,P)-cis-1a-d and (R,R)-(P,P)-cis-7
in the addition of silyl ketene acetal 9 to 1-chloroisochroman 8.
Motor (10 mol%)
wavelength (nm)
wavelength (nm)
O
O
OTBS
OiPr
9
(5 equiv)
b)
OiPr
Cl
8a
10a
O
MTBE (0.2 M), 36 h
(P,P)-trans-1b
PSS ratio = 90:10
(M,M)-cis:(P,P)-trans-1b
Entry
Motor
T (ºC)
conversion (%)a
e.r.b
1
2
(R,R)-(P,P)-cis-7
(R,R)-(P,P)-cis-1a
(R,R)-(P,P)-cis-1b
(R,R)-(P,P)-cis-1c
(R,R)-(P,P)-cis-1d
(R,R)-(P,P)-cis-1b
(R,R)-(P,P)-cis-1b
(R,R)-(P,P)-cis-1b
-70
-70
-70
-70
-70
-80
-80
-80
20
17
50:50
81:19
86:14
84:16
80:20
88:12
90:10
90:10
(M,M)-cis-1b
3
65
(P,P)-cis-1b
4
31
5
47
6
46
0
ppm)
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.
7c
8c,d
40
Figure 1. a) UV-vis spectra (degassed THF, 10−5 M) and b) 1H NMR spectra
(CD2Cl2) of (P,P)-trans, (M,M)-cis, and (P,P)-cis-1b. c) CD spectra at 20°C in
degassed THF (10−5 M) of (M,M)-cis, and (P,P)-cis-1b before and after the
addition of 10 equiv TBACl.
91(81)e
aDetermined by 1H NMR. bDetermined by chiral HPLC. c[8a] = 0.15 M.
dReaction time d. eIsolated yield in parentheses. TBS
tert-
butyldimethylsilyl. MTBE = methyl tert-butyl ether.
=
5
=
We next evaluated the stereochemical outcome of the reaction
catalyzed by each of the interconvertible isomers of 1b generating
1-chloroisochroman derivatives 8 in situ from the corresponding
1-methoxyisochromans 11 (Scheme 3). The reaction of 8a in the
presence of (R,R)-(P,P)-trans-1b under the previously optimized
reaction conditions led to the formation of nearly racemic 10a (e.r.
= 51:49), which was isolated in 48% yield. The reaction catalyzed
Having established 1b as the most efficient catalyst for the
formation of 10a, its photoswitching behavior was next examined
by UV-vis spectroscopy (Figure 1). Irradiation of a solution of
(R,R)-(P,P)-trans-1b in THF (312 nm) promoted its isomerization
to (R,R)-(M,M)-cis-1b (Figure 1a). The presence of clear
isosbestic points is indicative of a unimolecular process (see
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10.1002/anie.201913054
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RESEARCH ARTICLE
by (R,R)-(M,M)-cis-1b afforded as expected (S)-10a in 90% yield
and 10:90 e.r. Gratifyingly, the use of pure (R,R)-(M,M)-cis-1b,
which results from the irradiation of (R,R)-(P,P)-trans-1b with 312
nm light, promoted the preferential formation of the opposite
enantiomer, giving rise to (R)-10a in 47% yield and 74:26 e.r.
Notably, the conversion reached after 5 d was significantly lower
in the presence of both (R,R)-(P,P)-trans-1b and (R,R)-(M,M)-cis-
1b as compared to (P,P)-cis-1b, which was consistently observed
for other 1-chloroisochroman derivatives. In the case of (R,R)-
(P,P)-trans-1b, the lower activity can be rationalized based on the
absence of cooperativity between the two oligotriazole branches,
whereas in the case of (R,R)-(M,M)-cis-1b, we attribute this effect
to a less optimal geometry for chloride binding. Differences in
catalytic activity between (M,M)-cis and (P,P)-cis motors had
been previously observed in our group in the context of
stereodivergent catalysis.[8] In a similar vein, methyl-substituted
derivatives 11b and 11c gave rise to racemic 10b and 10c in the
presence of (R,R)-(P,P)-trans-1b, whereas the use of (R,R)-
(M,M)-cis-1b and (R,R)-(P,P)-cis-1b promoted the formation of
BCl3, CH2Cl2
0 ºC to rt, 2 h
1b (10 mol%)
R
R
R
O
O
O
OTBS
OiPr
9
(5 equiv)
OMe
Cl
CO2iPr
11
8
10
MTBE (0.15 M)
-80 ºC, 5 d
O
O
O
CO2iPr
trans-1b:
48%, 51:49 e.r.
(M,M)-cis-1b:
47%, 74:26 e.r. (R)
(P,P)-cis-1b:
90%, 10:90 e.r. (S)
Δee = 128%
CO2iPr
10a
10b
10c
CO2iPr
trans-1b:
trans-1b:
58%, 50:50 e.r.
(M,M)-cis-1b:
45%, 80:20 e.r. (R)
(P,P)-cis-1b:
82%, 9:91 e.r. (S)
Δee = 142%
53%, 50:50 e.r.
(M,M)-cis-1b:
41%, 65:35 e.r. (R)
(P,P)-cis-1b:
81%, 14:86 e.r. (S)
Δee = 102%
O
CO2iPr
trans-1b:
O
F
10d
Cl
10e
O
CO2iPr
trans-1b:
42%, 50:50 e.r.
(P,P)-cis-1b:
50%, 5:95 e.r. (S)
10f
CO2iPr
trans-1b:
36%, 50:50 e.r.
(P,P)-cis-1b:
50%, 50:50 e.r.
(P,P)-cis-1b:
50%, 25:75 e.r. (S)
enantioenriched products.
In the case of 10b, opposite
60%, 18:82 e.r. (S)
enantiomers were preferentially obtained in 65:35 e.r. and 14:86
e.r. through the use of (R,R)-(M,M)-cis-1b and (R,R)-(P,P)-cis-1b,
respectively, which corresponds to 102% Δee. Likewise, reversal
of stereoselectivity in the formation of 10c was achieved,
obtaining a remarkable 142% Δee when switching from (R,R)-
(M,M)-cis-1b to (R,R)-(P,P)-cis-1b. Unfortunately, (R,R)-(M,M)-
cis-1b was not able to promote the addition of 9 to 8d-f under the
optimized reaction conditions and only starting material was
recovered after 5 d. This lack of reactivity is attributed to the low
solubility observed for its adducts with the corresponding 1-
chloroisochroman derivatives at the optimized reaction
temperature. Conducting the reaction at higher temperatures
(over -20 ºC) led to conversion to the product, although not
surprisingly, the levels of enantioinduction were very poor.
Nonetheless, in all those cases switching between (R,R)-(P,P)-
trans-1b and (R,R)-(P,P)-cis-1b allowed for control on the
stereochemical outcome in the formation of 10d-f, which were
obtained as racemates in the presence of (R,R)-(P,P)-trans-1b
and as enantioenriched products when (R,R)-(P,P)-cis-1b was
used, in up to 95:5 e.r. in the case of 10e.
Scheme 3. Stereodivergent addition of silyl ketene acetal
chloroisochromans 8 catalyzed by motors 1b.
9 to 1-
Conclusion
In summary, we have developed a photoresponsive chiral
oligotriazole anion receptor based on a molecular motor core,
which promotes the stereodivergent addition of silyl ketene acetal
nucleophiles to oxocarbenium ions via anion binding catalysis.
The stereoselectivity of this transformation can be modulated
through the light- and heat-driven rotation around the double bond
axis of the molecular motor, which functions as a multistage chiral
switch. Products 10 were obtained as racemates in the presence
of the (P,P)-trans state, and up to 80:20 e.r. with (M,M)-cis state
and 5:95 e.r. with (P,P)-cis state, promoting each of the cis states
the formation of opposite enantiomers. The unidirectionality of
molecular motors translates into a defined sequence of isomers
formed as one moves forward in the cycle. In our particular case,
the cycle (R,R)-(P,P)-trans-1b - (R,R)-(M,M)-cis-1b - (R,R)-(P,P)-
cis-1b gives rise to addition products in the order (R,S) racemic -
R enantiomer - S enantiomer. Starting from the (S,S)-configured
motor would invert the sequence resulting in (R,S) racemic - S
enantiomer - R enantiomer. Although with lower absolute
enantioselectivities than those reported for non-switchable
organocatalysts, reversal of enantioselectivity with up to 142%
Δee was achieved in this work starting from a single enantiomer
of the catalyst through simple irradiation and heating steps. These
results highlight the potential of helicates with switchable
configuration for asymmetric synthesis and open up new avenues
not only for future applications in the field of photoswitchable
catalysis but also in the arena of responsive supramolecular
assemblies.
Acknowledgements
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10.1002/anie.201913054
Angewandte Chemie International Edition
RESEARCH ARTICLE
Financial support from the Ministry of Education, Culture and
Science (Gravitation Program 024.001.035), the Ramón Areces
Foundation (Postdoctoral fellowship to R.D.), and the European
Research Council (Advanced Investigator Grant No. 694345 to
B.L.F.) are gratefully acknowledged.
Keywords: molecular motors
•
anion binding catalysis •
photoswitches • stereodivergent catalysis • oligotriazoles
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10.1002/anie.201913054
Angewandte Chemie International Edition
RESEARCH ARTICLE
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RESEARCH ARTICLE
Ruth Dorel, Ben L. Feringa*
hν, Δ
Page No. – Page No.
hν
Δ
Stereodivergent Anion Binding
Catalysis with Molecular Motors
hν’
R, S (racemic)
R
S
A photoresponsive chiral catalyst based on an oligotriazole-functionalized unidirectional molecular motor has been developed for
stereodivergent anion binding catalysis. The motor function controls the helical chirality of supramolecular assemblies with chloride
anions, which by means of chirality transfer enables the enantioselective addition of a silyl ketene acetal nucleophile to oxocarbenium
cations. Reversal of stereoselectivity (up to 142% Δee) was achieved through rotation of the motor core induced by photochemical
and thermal isomerization steps.
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